you mean like.. email?
Not sure how email solves that problem.
I mean, to be fair, even Facebook won't necessarily tell you about an event unless the event creator pays them money to promote it. It's actually not a great platform for small-scale publicity, and _only_ publicizing an event on Facebook is clearly a mistake for a number of reasons. But despite its many virtues, email doesn't actually address GP's case at all.
There isn't a passive tool that would solve this other than getting people's email or mailing address, and that's absolutely fine since if you wanted to go to these reunions and be invested in these events, you can send an email to the school and get yourself on this contact list.